The present invention relates to an apparatus to record or reproduce recording information in an optical information recording medium and to an objective lens and a pickup apparatus for use in the apparatus. Further, the present innovation relates to a near field type optical disk recording reproducing apparatus in which a distance between an objective lens and a recording surface of an optical disk is smaller than an order of a wavelength of light and to an objective lens and a pickup apparatus for use in the apparatus. In particular, the present invention relates to an recording reproducing apparatus in which NA (numerical aperture) is larger and a high density recording is conducted.
In a high density recording reproducing apparatus in which an optical disk is used as an optical information recording medium, an apparatus using a near field type optical system is suggested.
In such an objective lens in the near field type optical system, U.S. Pat. No. 5,125,750 discloses an optical system in which two pieces of separated lenses having a uniform refractive index are used, in particular, a hemisphere lens is used as a lens at the optical disk side.
In this optical system, since a precise optical system having a larger NA is constructed with the two pieces of lenses, many manufacturing processes are needed for assembling and adjustment. Further, a minute size lens is produced by polishing. As a result, the cost for this optical system becomes very high.
Further, in a recording reproducing apparatus for various optical information recording medium such as a compact disk (CD), a digital video disk (DVD), a mini disk (MD), and a magneto-optics disk (MO), due to the reason that the cost may be reduced, an objective lens made of plastic is used. Also, as an objective lens for use in a near field type optical system, there may be a possibility that a plastic lens is used.
However, although the objective lens made of plastic may be produced at low cost, there may be a problem that a focusing point is shifted due to a change in temperature.
With a simple structure, the present invention is to obtain an optical system for recording or reproducing a high density optical information recording medium, wherein the optical system has an excellent image forming performance as a near field type optical system and can be easily assembled and adjusted.
Further, an object of the present innovation is to provide an objective lens for use in an optical pickup device, wherein the objective lens is made of plastic material and a shift of a focusing point due to a change in temperature is small, in particular, to provide an objective lens for use in an optical pickup apparatus, wherein a shift of a focusing point due to a change in temperature is kept within a depth of focus even if a numerical aperture at the image side is larger than 0.5.
The above object may be attained by the following structures.
(1) A near field type optical disk recording/reproducing apparatus, comprises
a light source;
an objective single lens to converse light from the light source to a focusing point;
an optical disk; and
a rotating device to rotate the optical disk;
wherein the objective lens has a first surface at the light source side and a second surface at the focusing point side, and the first surface is a curved surface on which a diffraction surface is provided.
(2) In the near field type optical disk recording reproducing apparatus of (1), the first surface is an aspherical surface.
(3) In the near field type optical disk recording reproducing apparatus of (1), the second surface is a flat surface.
(4) In the near field type optical disk recording reproducing apparatus of (1), the diffraction surface is notched in a form of a ring.
(5) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
(1xe2x88x921/n) less than R0/d less than (1+1/n)
xe2x80x83wherein n is a refractive index of the objective lens, R0 is a paraxial curvature radius of the objective lens, and d is a thickness of the objective lens on the optical axis.
(6) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
xe2x80x83(1xe2x88x921/n) less than R0/(nxc2x7f) less than (1+1/n)
xe2x80x83wherein n is a refractive index of the objective lens, R0 is a paraxial curvature radius of the objective lens, and f is a focal length of the objective lens.
(7) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
0.3xc3x97(1xe2x88x923/xcexdd) less than (nxe2x88x921)/R0 less than 1/f
xe2x80x83wherein n is a refractive index of the objective lens, R0 is a paraxial curvature radius of the objective lens, f is a focal length of the objective lens, and xcexdd is Abbe""s number of the objective lens.
(8) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
0.4xc3x97(1xe2x88x923/xcexdd) less than (nxe2x88x921)/R0 less than 1.2xc3x97(1xe2x88x923/xcexdd)
xe2x80x83wherein n is a refractive index of the objective lens, R0 is a paraxial curvature radius of the objective lens, f is a focal length of the objective lens, and xcexdd is Abbe""s number of the objective lens.
(9) In the near field type optical disk recording reproducing apparatus of (1), the following formula is satisfied,
xcexdd less than 40
xe2x80x83wherein xcexdd is Abbe""s number of the objective lens.
(10) In the near field type optical disk recording reproducing apparatus of (1), further comprising a collimator lens, chromatic aberration of the objective lens is corrected excessively and chromatic aberration of the collimator lens is corrected insufficiently, whereby chromatic aberration of the entire optical system is corrected properly.
(11) A pickup apparatus for use in a near field type optical disk recording reproducing apparatus, comprises
a light source; and
an objective single lens to converse light from the light source to a focusing point;
wherein the objective lens has a first surface at the light source side and a second surface at the focusing point side, and the first surface is a curved surface on which a diffraction surface is provided.
(12) An objective lens for use in a near field type optical disk recording reproducing apparatus, wherein the objective single lens converses light from a light source to a focusing point, comprises
a first surface at the light source side, and
a second surface at the focusing point side, wherein the first surface is a curved surface on which a diffraction surface is provided.
(13) In a near field type optical disk optical system, the optical system is characterized in that an objective lens is a single lens, its light source side is a curved surface, a ring of a diffracting lens is notched on the curved surface.
(14) In the optical system of (13), the curved surface is an aspherical surface.
(15) In the optical system of (13), the focusing point side of the objective lens is a flat surface.
(16) In the optical system of (13), the following formula is satisfied:
(1xe2x88x921/n) less than R0/d less than (1+1/n)
xe2x80x83wherein the curved surface is an aspherical surface, its paraxial curvature radius R is R0 a thickness of the single lens on the optical axis is d, and a refractive index is n.
As stated above, in a near field type optical disk optical system, the optical system of the present invention comprises an objective lens which is a single lens whose light source side is a curved surface, preferably, an aspherical surface. It may be preferable that a ring of a diffracting lens is notched on the curved surface. It may be preferable that a surface at the focusing point side is a flat surface. It may be preferable that the objective lens is shaped in a flat convex lens as a whole.
If the objective lens of the near field type optical disk optical system is formed by a general flat convex lens which does not includes a diffracting lens on its convex surface, and if an aspherical surface is used as the convex lens, spherical aberration can be corrected well and a image forming capability on the optical axis can keep a good condition having no aberration. However, for a light flux in field angle of xc2x11xc2x0, the image forming capability is deteriorated greatly. This is because, with the flat convex lens having only refraction, a sine condition is very bad.
To counter the deterioration, the function to convert an incident light into a conversed light on the convex surface at its light source side is allotted not only to the refraction, but also to a diffracting power, whereby the curvature of the convex surface can be made loose and the sine condition becomes good. As a result, even for a light flux in field angle of xc2x11xc2x0, the good image forming capability can be maintained.
(17) An optical information recording medium recording reproducing apparatus, comprises
a light source; and
a objective lens of a single lens to converse light from the light source to a focusing point;
wherein the objective lens is made of plastic and comprises a first surface and a second surface opposite to the first surface, and at least one of the first surface and the second surface includes a diffraction surface, and
wherein the objective lens satisfies the following formula:
0.4xe2x88x92(63 "psgr"xc2x7xcex/NA2)xe2x89xa6"psgr"D/"psgr"xe2x89xa60.8+(63 "psgr"xc2x7xcex/NA2)
xe2x80x83where "psgr" is a power of the entire system of the objective lens, "psgr"D is the sum of the power of the diffraction surface of the first surface and the power of the diffraction surface of the second surface, xcex is a wavelength of the light source, and NA is a numerical aperture at the image side of the objective lens.
(18) In the optical information recording medium recording reproducing apparatus of (17), the diffraction surface is notched in the form of a ring.
(19) In the optical information recording medium recording reproducing apparatus of (17), the diffraction surface is a blaze type diffraction surface.
(20) In the optical information recording medium recording reproducing apparatus of (19), the numerical aperture at the image side of the objective lens is larger than 0.5, the diffraction surface is provided at the longer conjugate side, and the following formula is satisfied:
xe2x80x830.2NAxe2x88x92(32"psgr"xc2x7xcex/NA)xe2x89xa6mxc2x7xcexPminxe2x89xa62.6NA+(200"psgr"xc2x7xcex/NA)
xe2x80x83where Pmin is the minimum pitch of the blazes of the objective lens in a direction perpendicular to the optical axis, and m is a diffraction order of the blazes.
(21) In the optical information recording medium recording reproducing apparatus of (19), the diffraction surface is shaped in the form of blazes such that the diffraction efficiency becomes the largest for the diffracting light having an order number larger than the second order.
(22) A pickup apparatus for use in an optical information recording medium recording reproducing apparatus, comprises
a light source; and
a objective lens of a single lens to converse light from the light source to a focusing point;
wherein the objective lens is made of plastic and comprises a first surface and a second surface opposite to the first surface, and at least one of the first surface and the second surface includes a diffraction surface, and
wherein the objective lens satisfies the following formula:
0.4xe2x88x92(63"psgr"xc2x7xcex/NA2)xe2x89xa6"psgr"/"psgr"Dxe2x89xa60.8+(63"psgr"xc2x7xcex/NA2)
xe2x80x83where "psgr" is a power of the entire system of the objective lens, "psgr"D is the sum of the power of the diffraction surface of the first surface and the power of the diffraction surface of the second surface, xcex is a wavelength of the light source, NA is a numerical aperture at the image side of the objective lens.
(23) An objective lens for use in an optical information recording medium recording reproducing apparatus, comprises
a objective lens having a first surface and a second surface opposite to the first surface;
wherein the objective lens is made of plastic and at least one of the first surface and the second surface includes a diffraction surface, and
wherein the objective lens satisfies the following formula:
0.4xe2x88x92(63"psgr"xc2x7xcex/NA2)xe2x89xa6"psgr"/"psgr"Dxe2x89xa60.8+(63"psgr"xc2x7xcex/NA2)
xe2x80x83where "psgr" is a power of the entire system of the objective lens, "psgr"D is the sum of the power of the diffraction surface of the first surface and the power of the diffraction surface of the second surface, xcex is a wavelength of the light source, NA is a numerical aperture at the image side of the objective lens.
(24) In an objective lens of a single lens for use in a pickup for an optical recording reproducing apparatus, the objective lens for pickup is characterized in that the objective lens is made of plastic, a blazed type diffraction surface in which a saw tooth-shaped level difference is made in the form of a ring is provided on at least one of a first surface and a second surface, and the following formula is satisfied:
0.4xe2x88x92(63"psgr"xc2x7xcex/NA2)xe2x89xa6"psgr"D/"psgr"xe2x89xa60.8+(63 "psgr"xc2x7xcex/NA2)
xe2x80x83where "psgr" is a power of the entire system, "psgr"D is the sum of the power of the diffraction surface of the first surface and the power of the diffraction surface of the second surface, xcex is a wavelength, and NA is a numerical aperture at the image side.
(25) In the objective lens for pickup of (24), the numerical aperture at the image side of the objective lens is larger than 0.5, the diffraction surface is provided at the longer conjugate side, and the following formula is satisfied:
0.2NAxe2x88x92(32"psgr"xc2x7xcex/NA)xe2x89xa6mxc2x7xcexPminxe2x89xa62.6NA+(200"psgr"xc2x7xcex/NA)
xe2x80x83where Pmin is the minimum pitch of the blazes of the objective lens in a direction perpendicular to the optical axis, and m is a diffraction order of the blazes.
(26) In the objective lens for pickup of (24), the diffraction surface is shaped in the form of blazes such that the diffraction efficiency becomes the largest for the diffracting light having an order number larger than or equal the second order.